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Re: a question about list as an element in a tuple

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  Re: a question about list as an element in a tuple John O'Hagan <research@johnohagan.com> - 2014-02-19 18:14 +1100
    Re: a question about list as an element in a tuple Marko Rauhamaa <marko@pacujo.net> - 2014-02-19 10:51 +0200
      Re: a question about list as an element in a tuple Marko Rauhamaa <marko@pacujo.net> - 2014-02-19 10:54 +0200

#66667 — Re: a question about list as an element in a tuple

On Mon, 16 Dec 2013 11:30:13 +0800
liuerfire Wang <liuerfire@gmail.com> wrote:

> Just like below:
> 
> In [1]: a = ([], [])
> 
> In [2]: a[0].append(1)
> 
> In [3]: a
> Out[3]: ([1], [])
> 
> In [4]: a[0] += [1]
> ---------------------------------------------------------------------------
> TypeError                                 Traceback (most recent call
> last) <ipython-input-4-ea29ca190a4d> in <module>()
> ----> 1 a[0] += [1]
> 
> TypeError: 'tuple' object does not support item assignment
> 
> In [5]: a
> Out[5]: ([1, 1], [])
> 
> no problem, there is an exception. But a is still changed.
> 
> is this a bug, or could anyone explain it?
> 
> thanks.
> 

This thread from two years ago deals with this in some detail:

https://mail.python.org/pipermail/python-list/2012-February/619265.html

It's one of those things that is hard to resolve in a way that makes
sense in every situation. The weirdest part for me is this:

>>> t = ([],)
>>> l = t[0]
>>> l is t[0]
True
>>> l += [1]
>>> t[0] += [1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'tuple' object does not support item assignment

Whether there is an error or not depends on the name used for the
object!

--

John

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#66677

John O'Hagan <research@johnohagan.com>:

> The weirdest part for me is this:
>
>>>> t = ([],)
>>>> l = t[0]
>>>> l is t[0]
> True
>>>> l += [1]
>>>> t[0] += [1]
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: 'tuple' object does not support item assignment
>
> Whether there is an error or not depends on the name used for the
> object!

Nice catch! The += operator rebinds the reference even if the object
wouldn't change:

   >>> t = 1,
   >>> t[0] = t[0]
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   TypeError: 'tuple' object does not support item assignment

See also:

   >>> a = []
   >>> b = a
   >>> a = a + [1]
   >>> a is b
   False
   >>> a = []
   >>> b = a
   >>> a += [1]
   >>> a is b
   True

This behavior is not a bug, though. <URL:
http://docs.python.org/3.2/library/operator.html#inplace-operators>:

  for example, the statement x += y is equivalent to x =
  operator.iadd(x, y)

operator.iadd(x, y) modifies x in place and returns x. However, (<URL:
http://docs.python.org/3.2/library/operator.html#operator.add>) x + y is
dealt with by operator.add(x, y), which leaves x and y intact and must
return a new object.


Marko

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#66679

Marko Rauhamaa <marko@pacujo.net>:

> operator.add(x, y) [...] leaves x and y intact and must return a new
> object.

Well, if the addition doesn't modify x, the method can of course return
x.


Marko

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